Field of the Invention
The present invention relates to initial run-in control of a clutch of an automatic transmission.
Description of the Related Art
Japanese Patent Laid-Open No. 8-233090 describes a technique which performs control such that a torque phase after gear change (speed change) is started in a hydraulic transmission immediately changes to an inertia phase without decreasing the torque, thereby protecting a driver from feeling a shock at a gear change (shift shock).
Normally, multi-plate wet clutches for use in an automatic transmission of a vehicle each have a variation in friction coefficient (μ) of the clutch surface. Therefore, in a predetermined period during which an unused clutch absorbs a predetermined amount of heat, an initial fitting process (initial run-in) of heating the clutch surface is necessary. In a period during which this initial run-in is incomplete, the friction coefficient of the clutch surface tends to be small. Accordingly, if normal shift control is performed by assuming that initial run-in is complete, torque missing by which torque transmission is temporarily interrupted during the shift operation occurs. Consequently, the main shaft rotational speed of the transmission fluctuates during the shift operation, so an engine rotation increase (engine racing) occurs or shift shock worsens.
To solve these problems, engine racing can be suppressed by setting a high clutch surface pressure (coupling torque) during coupling if it is assumed that initial run-in is incomplete, but the shift shock often worsens. There is also a method of learning the initial friction coefficient of a clutch for which no initial run-in is performed. Since, however, a change in initial friction coefficient is abrupt and largely varies, no learning precision can be assured, so the method is not an effective solution. Furthermore, the initial friction characteristic of a clutch varies from one clutch to another, so it is necessary to take this individual difference into consideration as well.
In a period during which initial run-in is incomplete, therefore, it is necessary to promote initial run-in while performing control so as to suppress engine racing and shift shock during a shift operation.